1. Field of the Invention
The invention relates in general to a method of fabricating a capacitor, and more particularly, to a method of fabricating a dual cylindrical capacitor in a dynamic random access memory (DRAM).
2. Description of the Related Art
As the functions of microprocessor become more and more versatile, the program and the calculation run by a software becomes more and more sizeable. The storing capacitance of a memory is required to be larger and larger. FIG. 1 shows a circuit diagram of a memory cell in a DRAM device. A memory cell typically comprises a transfer transistor 10 and a storing capacitor 11. The source of the transistor 10 is connected to the corresponding bit line 12, where the gate is connected to the word line 13, and the drain is connected to the storing electrode 14, that is, the bottom electrode, of the capacitor 11. The plate electrode 15, also called the top electrode or the cell plate, of the capacitor 11 is connected to a power supply. The capacitor 11 further comprises a dielectric layer 16 between the bottom electrode 14 and the top electrode 15.
The capacitor plays as a heart of a DRAM, the more charges the capacitor can store, the less it is affected by noise, for example, soft error caused by an .alpha. particle, while accessing data. Furthermore, the frequency of refresh can be reduced. Currently, methods to increase capacitance of the capacitors are (1) to increase the dielectric constant of the dielectric layer, so that the charge stored in a unit area is increased; (2) to reduce the thickness of the dielectric layer; and (3) to increase the surface area of the capacitor.
Conventionally, a two dimensional capacitor, that is, a planar type capacitor, is used in a DRAM to increase the capacitance. The planar capacitor occupies a large surface area of the device. Therefore, in a device with high integration, this kind of capacitor can not be applied. A three dimensional capacitor, such as a stacked type, a trench type, or a cylinder type capacitor, is then developed to overcome the drawback of the planar type capacitor.
A single cylindrical capacitor has been developed for the purpose of increasing capacitance. However, as the device with a higher integration demands a further higher capacitance, to design a capacitor with increasing capacitance in the limited space without complicating the fabricating process becomes a trend in semiconductor manufacture and the relative field.